A taxonomic and ecological study of the phytoplankton of the estuaries of the Schelde (Belgium-The Netherlands) and the Elbe (Germany) revealed the presence of numerous species belonging to the diatom genus Thalassiosira. Its representatives were a dominant component of the phytoplankton spring bloom in these estuaries. A total of 13 species were identified. All of these were present in the Schelde, while only ten were found in the Elbe. Seven and five species respectively had previously not been reported from the Schelde and Elbe. A detailed light and electron microscopical description of these species is given. Most species had their distributional optimum in the polyhaline reaches of the estuaries. Community structure in these reaches was different from the one in the euhaline zone, indicating that the lower and middle reaches of the estuaries harbour specific assemblages which do not occur in neighbouring coastal environment. Only one species, Thalassiosira proschkinae, had its maximal abundance in the mesohaline zone and can thus be regarded as a truly brackish water species. Thalassiosira pseudonana was restricted to the tidal freshwater zone of the estuaries. Some taxa often appeared to be associated with sediment and detritus particles. The significance of this phenomenon is discussed.